Recent experimental results indicate that there are serious problems in forecasting planetary scales of motion. In contrast with predictability theory which suggests that the planetary scales are the most predictable, forecast experiments indicate that the long waves are predicted less accurately than the synoptic scales. The present work suggests that one of the causes of model long wave error is the spurious excitation of transient external large-scale Rossby modes. It was found that these modes can be excited by the imposition of an equatorial wall, or by the use of unsuitable data in the tropics. Paradoxically, the imposition of a wall north of the equator may tend to suppress these spurious Rossby modes. The excited external Rossby modes are relatively fast-moving and can have a substantial negative impact on midlatitude forecast skill after only 24 h of integration.